- Please try to reenable add-on in Administration - Add-ons - Manage add-ons, or restarting Jira
Also please make sure you use the right add-on version 2.9.x for JIRA 6 or 3.x for JIRA 7 or 4.x for JIRA 8, and there is no old version in Jira HOME, e.g. with command like:
find JIRA-HOME -name jira-timesheet-plugin*.jar | xargs rm
and then restart Jira and install plugin again.